2. Field of the Invention
The present invention relates to an organic laminated photosensitive material of the positive charging type and a process for the preparation thereof. More particularly, the present invention relates to an organic laminated photosensitive material of the positive charging type having an excellent sensitivity and a process for the preparation thereof.
2. Description of the Prior Art
A photosensitive material for electrophotography comprising an electroconductive substrate and a layer of an inorganic or organic photoconductor formed on the substrate has been widely used. Representative of this photosensitive material, there is known a so-called function-separated organic photosensitive material in which a charge-generating substance and a charge-transporting substance are combined by lamination or dispersion.
Most of known function-separated organic photosensitive materials are of the negative charging type, but photosensitive materials of the negative charging type involve a problem in that ozone is generated at the time of charging. Accordingly, organic photosensitive materials of the positive charging type are eagerly desired.
As the organic photosensitive material of the positive charging type, there is known an organic photosensitive material comprising a charge-generating layer laminated on a charge-transporting layer having a hole-transporting property. In the organic photosensitive material, if the thickness of the carrier-generating layer is not reduced, injection of charges is not sufficiently performed, and if the thickness of the charge-generating layer is reduced, the abrasion resistance is poor and the printing resistance is degraded.
As means for overcoming this defect, Japanese Patent Application Laid-Open Specification No. 92962/87 discloses a photosensitive material comprising a carrier-generating layer (charge-generating layer) comprising anthanthrone bromide as the carrier-generating substance, a carrier-transporting substance and a binder resin. In this photosensitive material, the same substance is commonly used as the hole-transporting substance in the charge-generating and transporting layer and the hole-transporting substance in the charge-transporting layer.
This known photosensitive material is significant in that by incorporating the charge-transporting substance in the charge-generating layer, injection of holes in the charge-transporting layer can be performed smoothly even if the topmost charge-generating layer is relatively thick. However, if the same substance is used as the hole-transporting substance in the charge-generating and transporting layer and the hole-transporting substance in the charge-transporting layer, when the charge-generating and transporting layer is formed by coating, dissolution of the hole-transporting substance of the lower charge-transporting layer into the coating liquid for the upper layer cannot be avoided and the concentration of the hole-transporting substance in each of the charge-transporting layer and the charge-generating and transporting layer cannot be strictly controlled. Especially in the case where the charge-generating and transporting layer is formed by the dip coating method, the dissolution of the hole-transporting substance of the lower layer is very disadvantageous for controlling the concentration of the coating liquid.
Whether it is easy or difficult to inject holes between the charge-generating and transporting layer and the charge-transporting layer has serious influences on the sensitivity of the final photosensitive material, and the standard for selection of the hole-transporting substances based on the easiness of injection of holes has not been established.